This invention relates to a reliable signalling apparatus in which a transmitter assembly having at least one transmitter impresses a signal current on the series connected inputs of several receivers in physically (spatially) separated receiver assemblies.
Such signalling apparatus is commercially available for the reliable transmission of signals. Impressed currents are frequently used instead of impressed voltages since the transmission of current, as opposed to the transmission of voltage, does not lead to error signals in the event of changes in the reference potential (ground) between transmitter and receiver. When several receivers are to be connected to a transmitter, the receivers can be connected in series. This simplifies the supervision of the transmitter and the receivers considerably. This arrangement has the disadvantage, however, that any interruption in the series circuit leads to the failure of all receivers. To avoid this, a separate transmitter can be provided for each receiver. Then, however, increased circuit cost and a correspondingly increased probability of failure must be tolerated. It also becomes substantially more difficult to control all transmitted and received signals so that they register.
It is therefore an object of the present invention to provide a signalling apparatus of the type mentioned at the outset in which the signal transmission is reliable while, at the same time, expenditure for circuitry is low.